Recently, flexible wiring substrates for mounting a liquid crystal driver thereon have been improved to allow wring patter to have finer pitches, in order to cope with an improvement of the liquid crystal driver to have more outputs. Moreover, size reduction of the flexible wiring substrates including a protective resin portion for protecting a semiconductor element in order to cope with an improvement of a semiconductor device to be lighter in weight, thinner in thickness, a shorter in length, and smaller in dimension.
At this moment, for mounting of liquid crystal driver IC, COF (Chip On Film) is getting more popular than TCP (Tape Carrier Package) because COF allows finer pitch of the wiring patter and more freedom as to bending positions (where to be bent).
Mounting of COF is carried out as follows.
as illustrated in FIGS. 8(a) and 8(b), a writing patterns 102 and 103, made of copper, are formed on a flexible film 101a made of a polyimide. Then, as illustrated in FIG. 9, a semiconductor chip 104 with a bump electrode 105 is bonded on the flexible film 101a. 
Next, a sealing resin to be an under-fill 106 is applied to fill and seal between the semiconductor chip 104 and the flexible wiring substrate 101. Then, a heat treatment is carried out thereby to cure the sealing resin.
By using a nozzle 141 jetting out the sealing resin therefrom in constant quantity, the application of the sealing resin for the under-fill 106 is carried out according to a resin application pattern predetermined in accordance with a shape of the semiconductor chip 104, as illustrated in FIGS. 10(a) and 10(b). Thereby, the sealing resin is introduced from four sides of the semiconductor chip 104. In this way, the sealing resin is introduced in a gap between the semiconductor ship 104 and the flexible wiring substrate. The sealing resin flows between the semiconductor ship 104 and the flexible wiring substrate by capillary phenomenon, thereby eliminating any empty space therebetween. As a result, fillet portions 106a and 106b are formed on whole sides of the semiconductor chip 104. Then, the flexible film 101a is cut out in a usage shape 109 (a shape in which the semiconductor device is supplied to a user) as illustrated in FIG. 8(a), the flexible, thereby producing an individual COF semiconductor 110 as illustrated in FIG. 8(b). The resin application pattern for the jetting out of the sealing resin for the under-fill 106 depends on a flowability of a resin to use. Therefore, the resin application pattern should have been such as to apply the resin along the four sides of the semiconductor chip 104 in order to fill the gap between the semiconductor chip 104 and the flexible wiring substrate 101 without air bubble therein. Moreover, the application of the sealing resin according to the resin application pattern causes a resin trace 106c, which has a thickness of 30 μm to 50 μm or more. With such a thick thickness, the resin remains clearly.
In order to cope with the improvement of the semiconductor device to be lighter in weight, thinner in thickness, shorter in length, and smaller in dimension, it is necessary to reduce not only the semiconductor chip itself but also a resin portion of the semiconductor chip.
COF semiconductor devices are advantageous in the greater freedom as to the bending portion. Regarding the freedom as to the bending portion, the smaller dimension of the resin portion is more preferable, because the resin portion is inflexible. If the resin sealing section was bent beyond its limit, such problems would be caused such as causing a crack in the sealing resin, pealing the sealing resin off from the flexible substrate.
In the conventional semiconductor device and its manufacturing method, the fillet portions 106a, 106b, and resin trace 106c, which are caused as a result of the formation of the under-fill 106, take a significant space, thereby forming a large resin portion. A mechanical design should be design such that the resin portion is not included in a bending region. Thus, a large resin portion becomes a large restriction against product miniaturization. Specifically, as illustrated in FIG. 10(b), the fillet portions 106a and 106b having the same width from the semiconductor chip 104, are in a surrounding of the semiconductor chip 104. Moreover, as illustrated in FIG. 9, the thickness of the resin trace 106c is so thick that it has a thickness in a range of 30 μm to 50 μm. Therefore, the resin trace 106c also forms a inflexible portion.
On the other hand, electric insulating resistance between the adjacent wiring patterns 102 and 102, and between the adjacent wiring patterns 103 and 103 is a factor most influential on reliability of the flexible wiring substrate 101 that is improved to have the wiring patterns 102 and 103 deposited with finer pitches. FIG. 11 illustrates a case where air bubbles 151 and 152 are entrapped when the sealing resin is introduced between the semiconductor chip 104 and the flexible wiring substrate 101 in order to form the under-fill 106 serving to protect the semiconductor 104, and fail to escape outside the sealing resin before the resin is cured, thereby remaining (a) below the semiconductor chip 104, (b) in contact with the bump electrode 105, (c) between the wiring patterns 102, or (d) between the wiring patterns 103. In this case, a space is produced in contact (a) below the semiconductor chip 104, (b) in contact with the bump electrode 105, (c) between the wiring patterns 102, or (d) between the wiring patterns 103. Moisture from outside, or residual ion component remaining in the resin would come and remain in the space. Migration would easily occur in such a space in which water or residual ion component is kept. The migration leads to deterioration of the electric insulating resistance between the terminals.
Moreover, the conventional sealing resin has a high viscosity that gives the conventional sealing resin a poor flowability. Due to the poor flowability, the conventional sealing resin should be applied along the four sides of the semiconductor chip 104. Otherwise, the fillets would be uneven as illustrated in FIG. 12, where only one fillet section 106a is formed but the fillet section 106b is not formed. Such failing in filling, such as leaving the wiring patterns 102 or 103 uncovered leads to a qualitative problem.
The technique disclosed in Japanese Unexamined Patent Publication, Tokukai, No. 2003-174045 (published on Jun. 20, 2003) includes the step of defoaming after the step of applying the sealing resin, in order to prevent the air gap in the under-fill. In the step of applying the sealing resin, the sealing resin is used whose viscosity is approximately 100 cp (1000 mPa·S) at 25° C. and which is heated up to 50° C. so as to lower the viscosity to approximately 250 cp (250 mPa·S). The application of sealing resin is followed by leaving the sealing resin for 120 sec. Then, the step of defoaming is carried out by attaching a tool to a surface of the flexible wiring substrate for 5 sec, the tool being heated up to a temperature in a range of 140° C. to 200° C. The application of the sealing resin is carried out along the four edges or periphery of the semiconductor chip. Moreover, the viscosity of the sealing resin is 1000 mPa·S at 25° C., which is so high to give the sealing resin a poor flowability. In order to lower the viscosity, the sealing resin is heated up to 50° C. However, if the heating time became too long, curing reaction would take place thereby increasing the viscosity of the sealing resin. This results in a lower flowability and shorter pot life of the sealing resin, thereby deteriorating workability.